(a) Field of the Invention
The present invention relates to a telephoto zoom lens system and, more particularly, to a telephoto zoom lens system which is compact in size and low in price and has a zoom ratio of about 3x.
(b) Description of the prior art:
A so-called afocal type zoom lens system is known as a telephoto zoom lens system, said afocal type zoom lens system comprising a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive or negative refractive power and a fourth lens group having positive refractive power and being arranged to vary the focal length and to compensate the image position by moving the second and third lens groups along the optical axis. However, said type of zoom lens system has a disadvantage that it is difficult to make the zoom ratio large because the first and fourth lens groups are kept fixed in respect to the image surface at the time of zooming. Besides, when the refractive power of the second lens group is made strong in case of said type of zoom lens system, it is possible to make the overall length of the lens system short. However, to correct aberrations favourably in that case, the number of lenses constituting the lens system should be made large. In other words, said type of zoomlens system has a disadvantage that it is impossible to make the lens system compact and, at the same time, to make the price low.
As a method to make the overall length of a zoom lens system short without making the refractive power of the second lens group strong, it is known to move the first lens group at the time of zooming, i.e., to arrange the lens system in such a way so that the overall length of the lens system varies at the time of zooming. When the above-mentioned type of zoom lens system is arranged so that the overall length thereof becomes satisfactorily short when the lens system is set to the shortest length, it is convenient for carrying the lens system. In that case, however, the structure of the lens mount becomes complicated because the number of movable lens groups increases. Moreover, the number of lenses constituting the lens system does not become so different compared with the afocal type zoom lens systems. Therefore, the cost of production is not low.
To solve the above-mentioned problems, a prior art device is provided by Japanese published unexamined patent applications Nos. 82432/78, 117514/83, 19931/83, etc. In this case, the zoom lens system is arranged to comprise three lens groups, i.e., a first lens group G.sub.1, a second lens group G.sub.2 and a third lens group G.sub.3, as shown in FIG. 1 and to be zoomed by moving the first lens group G.sub.1 and the second lens group G.sub.2. This lens system is arranged so that the number of lenses constituting the lens system is small, i.e., nine to eleven, and so that the lens system is compact in size and low in price. However, in said known zoom lens system, the stop is kept fixed in respect to the image surface together with the third lens group G.sub.3 and, therefore, the position of the entrance pupil varies largely at the time of zooming. As a result, variation of distortion becomes large and it is difficult to ensure satisfactory intensity of light in the marginal portion when the lens system is set to the teleposition. This is because the position of the entrance pupil is decided by the distance between the stop and the first lens group and, therefore, if the stop is kept fixed in respect to the image surface as described above, the position of the entrance pupil varies largely when the first lens group G.sub.1 is moved. As a result, the height of principal ray that passes the first lens group in the wide position becomes largely different from said height in the teleposition and, consequently, distortion varies. Especially, pincushion distortion becomes large in the teleposition, and it is difficult to correct it. Besides, lower rays are partially ecilipsed by some of lenses when the lens system is set to the teleposition and, therefore, it is difficult to ensure satisfactory intensity of light in the marginal portion.
Zoom lens systems disclosed by Japanese published unexamined patent applications Nos. 30709/83, 88717/83, 80143/79 (Japanese published examined patent application Nos. 11608/83), 156912/80 and 14213/81 are respectively known as zoomlens systems that resemble the zoom lens system according to the present invention. However, in cases of said known zoom lens systems, the lens configuration is complicated. Especially, the number of lenses constituting the first lens group is large, and this is not preferable for making the lens system compact. Moreover, in cases of said known zoom lens systems, the focal length in the wide position is shorter than the focal length of standard lens systems.